CLIMET INSTRUMENTS CI-154, User Manual, Page: 78 / 98

CI-154 User’s Manual
Page 68
4.2.1 The airflow system
The CI-154 uses a patented (USRE 37,353E, 5,515,164 ), regulated airflow system.
A brushless DC blower, acting as a vacuum source, pulls air through the sensor. An
airflow sensor generates a signal used in a closed loop circuit to control airflow. This
circuit controls the speed of the blower motor by producing a variable voltage, which
it applies to the blower motor. Any restriction on the inlet (such as placing a filter
over the inlet) will be detected as lower airflow, and the control circuit will increase
the voltage that drives the blower. This can be heard as a revving up of the blower
as it adjusts the airflow. This system also compensates for declining power when
operated with battery power.
Replacing the traditional pump with a blower made a 1 CFM flow rate possible with
the low power consumption needed to make the particle counter portable. This
system has proven itself since Climet first introduced it in 1993, which changed the
definition of portable for particle counters.
The inlet nozzle shapes the air stream to match the width of the laser beam. The
shape of the nozzle also establishes the velocity of the particles as they pass
through the laser beam.
To prevent the output of the blower from adding particles to the room, the airflow
first exhausts into a HEPA filter before being discharged through a port in the upper
left corner of the back panel. The emissions from the particle counter are tested
during manufacture to assure that each instrument passes the requirements for an
ISO Class 3 Environment.
4.2.2 The light source
The CI-154 uses a laser diode light source. The superior light collecting properties of
Climet’s optical system allows the laser diode to be driven at a much lower level
which adds to the life of the laser diode. Lenses are used to shape the laser beam. A
light trap at the opposite end of the sensor absorbs the beam, so that it will not
create stray light that would obscure smaller particles.
The output of the laser diode is regulated by feedback circuitry that detects the light
output from the laser diode and adjusts the drive current to maintain a constant
light output. This keeps the light output stable over the life of the laser diode, and
contributes to the stability of the sensor calibration between calibration intervals.
4.2.3 The optical system
The optical system of Climet particle counters have characteristics that provide
efficient light collection, a monotonic particle response curve (i.e., one that has a
slope that is always positive), and accuracy traceable to NIST.
The key to Climet’s optical system is an ellipsoidal mirror that provides a wide view
angle. This has two advantages. Because it collects light over a wide angle, it is less
sensitive to the refractive index of the particle. This increases accuracy when
counting real particles, as compared to calibration standards. Because of this wide
collection angle, it also collects light more efficiently, allowing it to see smaller
particles with less light energy. This allows the laser diode to be operated below full
power, helping to prolong the life of the laser diode.
As a particle approaches the wavelength of the light source, the angles of the light
scattered by the particle changes from particles with a diameter somewhat greater
than the wavelength. In extreme cases, with poorly designed optical systems, the